People with Prader-Willi syndrome (PWS) display variable responses to visual-visual learning and memory tasks, sometimes functioning better than matched controls and at others, performing less well on such tasks. Approximately 30% of people with PWS have been reported to have misrouting of visual pathways at the optic chiasm, which could be related to visual processing problems. In addition, people with PWS have very serious problems related to food motivation. It is possible these two sources of variability in cognitive performance may have been confounded in some situations in which performance of people with PWS have been evaluated. The present series of studies in an attempt to determine the role of food motivation versus visual memory and visual cognitive organizational ability in people with PWS as compared with matched controls. In addition, these studies will attempt to clarify whether visual-visual learning is easier or more difficult in people with PWS compared with matched controls. Thirty four children, adolescents, and adults with PWS and 34 controls matched for MA, CA, and percent of ideal body weight, will be evaluated for the role of food motivation in short term memory and relations learning. In the first study, the effects of three sets of food-related variables on visual short term memory will be examined: (a) testing several hours after the last meal versus testing immediately following a meal; (b) the use of stimuli in the memory task that involves food versus non-food related items, and (c) the use of food incentives versus no food incentives for correct matches in the memory task. The second study will examine the role of food nutrition in relational learning, in which food versus non-food rewards will be used for correct responses in matching to sample, and in which food items are used as members of stimulus classes to be acquired. The third study will examine the degree to which auditory-visual, versus visual-visual relational learning is acquired by people with PWS compared with matched controls, with and without differential consequences associated with each stimulus class. The fourth study will evaluate differences in ability to acquire visual- visual relations in which the order in which stimuli are presented is the relevant stimulus dimension. Such ordinal relations are important in concepts of causality and are a prerequisite to reading and other more complex skills. If there are differences in visual-visual, versus auditory-visual relational learning, we will attempt to determine post hoc whether these differences are related to misrouting of the visual pathways, or whether these differences are related to the perceptual differences previously reported.